Soft handoff in OFDMA system

ABSTRACT

Soft handoff in an OFDMA system is disclosed. If the pilot signal strength for a base station exceeds the defined threshold, the base station is added to an active set list. Subcarriers in a plurality of orthogonal frequency division multiplexing (OFDM) symbols are divided and allocated into subchannels. The OFDM symbols are divided and multiplexed. A soft handoff zone with a first dimension of the subchannels and a second dimension of the divided and multiplexed OFDM symbols is defined. The soft handoff zone has subcarriers with a subchannel definition, for example, an identical permutation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/221,579, filed on Aug. 30, 2011, now U.S. Pat. No. 8,249,024entitled, “SOFT HANDOFF IN OFDMA SYSTEM”, invented by Jianglei Ma etal., which is a divisional of U.S. patent application Ser. No.11/630,473 (now U.S. Pat. No. 8,023,466), whose §371(c) date was Jan.23, 2008, entitled “SOFT HANDOFF IN OFDMA SYSTEM”, which is the U.S.National Stage of International Application No. PCT/CA2005/000970, filedon Jun. 22, 2005, which claims the benefit of priority to U.S.Provisional Application No. 60/581,356, filed on Jun. 22, 2004. All ofthe above-identified applications are hereby incorporated by referencein their entireties as though fully and completely set forth herein.

FIELD OF THE INVENTION

The present invention relates to the delivery of data via a wirelessconnection and, more particularly, to the accurate delivery of data athigh rates via a wireless connection.

BACKGROUND OF THE INVENTION

Recent growth in demand for broadband wireless services enables rapiddeployment of innovative, cost-effective, and interoperable multi-vendorbroadband wireless access products, providing alternatives to wirelinebroadband access for applications such as telephony, personalcommunications systems (PCS) and high definition television (HDTV). Atthe same time, broadband wireless access has been extended from fixed tomobile subscriber stations, for example at vehicular speed. Though thedemand for these services is growing, the channel bandwidth over whichthe data may be delivered is limited. Therefore, it is desirable todeliver data at high speeds over this limited bandwidth in an efficient,as well as cost effective, manner.

In the ever-continuing effort to increase data rates and capacity ofwireless networks, communication technologies evolve. An encouragingsolution for the next generation broadband wireless access deliveringhigh speed data over a channel is by using Orthogonal Frequency DivisionMultiplexing (OFDM). The high-speed data signals are divided into tensor hundreds of lower speed signals that are transmitted in parallel overrespective frequencies within a radio frequency (RF) signal that areknown as subcarrier frequencies (“subcarriers”). The frequency spectraof the subcarriers may overlap so that the spacing between them isminimized. The subcarriers are also orthogonal to each other so thatthey are statistically independent and do not create crosstalk orotherwise interfere with each other. When all of the allocated spectrumcan be used by all base stations, the channel bandwidth is used muchmore efficiently than in conventional single carrier transmissionschemes such as AM/FM (amplitude or frequency modulation), in which onlyone signal at a time is sent using only one radio frequency, orfrequency division multiplexing (FDM), in which portions of the channelbandwidth are not used so that the subcarrier frequencies are separatedand isolated to avoid inter-carrier interference (ICI).

In OFDM, each block of data is converted into parallel form and mappedinto each subcarrier as frequency domain symbols. To get time domainsignals for transmission, an inverse discrete Fourier transform or itsfast version, IFFT, is applied to the symbols. The symbol duration ismuch longer than the length of the channel impulse response so thatinter-symbol interference is avoided by inserting a cyclic prefix or apredefined value for each OFDM symbol. Thus, OFDM is much lesssusceptible to data loss caused by multipath fading than other knowntechniques for data transmission. Also, the coding of data onto the OFDMsubcarriers takes advantage of frequency diversity to mitigate loss fromfrequency-selective fading when forward error correction (FEC) isapplied.

Another approach to providing more efficient use of the channelbandwidth is to transmit the data using a base station having multipleantennas and then receive the transmitted data using a remote stationhaving multiple receiving antennas, referred to as MultipleInput-Multiple Output (MIMO). The data may be transmitted such there isspatial diversity between the signals transmitted by the respectiveantennas, thereby increasing the data capacity by increasing the numberof antennas. Alternatively, the data is transmitted such that there istemporal diversity between the signals transmitted by the respectiveantennas, thereby reducing signal fading.

Wireless communication systems divide areas of coverage into cells, eachof which is served by a base station. A subscriber station willcontinuously monitor the signal strengths of the servicing base stationfor the current cell as well as for adjacent cells. The subscriberstation will send the signal strength information to the network. As thesubscriber station moves toward the edge of the current cell, theservicing base station will determine that the subscriber station'ssignal strength is diminishing, while an adjacent base station willdetermine the signal strength is increasing. The two base stationscoordinate with each other through the network, and when the signalstrength of the adjacent base station surpasses that of the current basestation, control of the communications is switched to the adjacent basestation from the current base station. The switching of control from onebase station to another is referred to as a handoff.

A hard handoff is a handoff that completely and instantaneouslytransitions from a first station to a second base station. Hard handoffshave proven problematic and often result in dropped calls. Wirelesssystems may incorporate a soft handoff, wherein when the subscriberstation moves from a first to a second cell, the handoff process happensin multiple steps. First, the subscriber station recognizes theviability of the second base station, and the network allows both thecurrent and adjacent base stations to carry the call. As the subscriberstation move closer to the second base station and away from the firstbase station, the signal strength from the first base station willeventually drop below a useful level. The subscriber station will theninform the network, which will instruct the first base station to dropthe call and let the second base station continue servicing the call.Accordingly, a soft handoff is characterized by commencingcommunications with a new base station before terminating communicationswith the old base station.

In orthogonal frequency division multiplexing access (OFDMA) systems,multiple users are allowed to transmit simultaneously on the differentsubcarriers per OFDM symbol. In an OFDMA/TDMA embodiment, for example,the OFDM symbols are allocated by time division multiplexing access(TDMA) method in the time domain, and the subcarriers within an OFDMsymbols are divided in frequency domain into subsets of subcarriers. Inother embodiment, to average inter-cell interference, different cellsmay use, for example, different permutations to generate subchannel.

It is therefore desirable to provide soft handoff to broadband wirelessaccess system employing OFDMA. Because different spreading code maskingis not available in OFDM transmission, the destructive interferencesbetween base stations transmitting the same signal can cause significantdegradation of performance.

It is further desirable to define a soft handoff zone with samepermutation wherein the base stations provide RF combining, interferenceavoidance, soft combining, or selection combination in the handoff area.It is further desirable to use multi-input, multi-output (MIMO) methodin a soft handoff of an OFDMA system. This MIMO method may apply to aplurality of base stations, each of the base stations has one, or morethan one antennas.

Accordingly, there is a need for an efficient soft handoff technique forOFDMA systems as well as a need to increase data rates and reduceinterference at cell borders. It is further desirable to provide softhandoff technique to a MIMO OFDMA system.

SUMMARY OF THE INVENTION

In downlink communications, each subscriber station constantly measuresall of the possible pilot signal strengths of transmissions fromadjacent base stations, identifies the strongest pilot signals, andcompares them against a defined threshold. If the pilot signal strengthfor a base station exceeds the defined threshold, that base station isadded to an active set list. Each subscriber station will notify thebase stations of their active set lists. If there is only one basestation in the active set list, said base station is singled out toservice the subscriber station. If there is more than one base stationon the active set list, a soft handoff is enabled between those basestations. The soft handoff condition will continue until only one basestation is on the active set list, wherein the lone base station willcontinue to serve the subscriber station. The soft handoff can beinitiated by the subscriber station, which will report the active setlist to the base station controller via the servicing base station. Thebase station controller will alert the base stations on the active setlist of the soft handoff. Notably, the base station controller canselect a sub-set of the base stations from the active set list toestablish the soft hand off. During soft handoff, all base stations onthe active set list will facilitate communications with the subscriberstation as defined below. Preferably, the base station controller keepstrack of all of the active set lists for the respective subscriberstations. The subscriber stations will keep track of their individualset lists.

Accordingly, by providing the set list to the base station controllerand the servicing base station, the subscriber station identifies thesole servicing base station or triggers a soft handoff (SHO) mode whenmultiple base stations appear on the active set list.

In orthogonal frequency division multiplexing access (OFDMA) systems,multiple users transmit simultaneously on the different subcarriers perOFDM symbol. In an OFDMA/TDMA embodiment, the OFDM symbols are allocatedby TDMA method in the time domain, and the subcarriers within OFDMsymbols are divided by OFDMA method in frequency domain into subsets ofsubcarriers, each subset is termed a subchannel. Each subchannel maycomprise the subcarriers from a plurality of OFDM symbols. Thesesubchannels are the basic allocation unit. The subchannel may be spreadover the entire bandwidth. This scheme achieves improved frequencydiversity and channel usage. In OFDMA, a transmit frame may be dividedinto uplink (UL) and downlink (DL) subframes in time division multiplex(TDD) mode. A zone is defined as a number of OFDMA symbols, in the DL orthe UL, that use the same subchannel definition, for example,permutation. A zone may be comprised of contiguous OFDM symbols. The DLsubframe or the UL subframe may contain more than one permutation zone.A soft handoff zone is defined for use in the handoff area with the samesubchannel definition between all active base stations.

Hence, the base stations in the active set can partition the time andfrequency resources of the OFDM signal. Accordingly, each base stationtransmits same signal through same channel resource (or samesubchannel). Preferably, an improved reception performance of thesubscriber stations is achieved through RF combining, soft combining, orselection combining. The base stations may further provide interferenceavoidance.

In a multi-input, multi-output system, base stations transmit space-timecode (STC) encoded data, and the subscriber stations providecorresponding STC decoding to recover the transmitted data. The STCcoding may be either space-time-transmit diversity (STTD),space-frequency-transmit diversity (SFTD) or space multiplexing (SM)coding. STTD coding encodes data into multiple formats andsimultaneously transmits the multiple formats with spatial diversity(i.e. from antennas at different locations). SM coding separates datainto different groups and separately encodes and simultaneouslytransmits each group. The subscriber station will separately de-modulateand decode the received data from each base station, and then combinethe decoded data from each base station to recover the original data. Inaccordance with one aspect of the present invention, there is provided amethod for facilitating soft handoffs in an orthogonal frequencydivision multiplexing access (OFDMA) system comprising the steps of:dividing subcarriers in a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols; allocating said divided subcarriers intosubchannels; dividing and multiplexing said plurality of OFDM symbols;defining a soft handoff zone comprising a first dimension of saidsubchannels and a second dimension of said divided and multiplexedplurality of OFDM symbols; said soft handoff zone having a firstplurality of subcarriers, said first plurality of subcarriers having anidentical subchannel definition; and transmitting said first pluralityof subcarriers in said soft handoff zone to a station. Preferably, thesubchannel definition is a subcarrier permutation. Preferably, thesubcarriers are divided in a frequency domain.

In accordance with another aspect of the present invention, there isprovided an orthogonal frequency division multiplexing access (OFDMA)system comprising: a base station controller adapted to schedule datafor a subscriber station during a soft handoff mode; a plurality of basestations operatively associated with said base station controller, eachbase station participating in said soft handoff being adapted to receiveat least a portion of scheduled data from said base station controller,and to transmit a first plurality of subcarriers to said subscriberstation; said first plurality of subcarriers being defined in a softhandoff zone comprising a first dimension of subchannels, saidsubchannels comprising said first plurality of subcarriers divided in afrequency domain; and a second dimension of divided and multiplexed OFDMsymbols comprising said first plurality of subcarriers; said firstplurality of subcarriers in said soft handoff zone having an identicalsubchannel definition.

In accordance with another aspect of the present invention, there isprovided a base station in an orthogonal frequency division multiplexingaccess (OFDMA) system comprising: subchannel definition logic adapted toprovide a subchannel definition to a first plurality of subcarriers;dividing and multiplexing encoding logic adapted to providedividing-multiplexing coding for a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols, said OFDM symbols comprising saidplurality of subcarriers; Inverse Fourier Transform (IFT) logic adaptedto provide an IFT on each of said first plurality of subcarriers togenerate said plurality of OFDM symbols, said first plurality ofsubcarriers being defined in a soft handoff zone comprising a firstdimension of subchannels, said subchannels comprising said firstplurality of subcarriers; and a second dimension of divided andmultiplexed plurality of OFDM symbols; said subcarriers in said softhandoff zone having an identical subchannel definition; and transmitcircuitry transmitting said first plurality of subcarriers for receptionby a subscriber station.

In accordance with another aspect of the present invention, there isprovided a subscriber station in an orthogonal frequency divisionmultiplexing access (OFDMA) system comprising: receive circuitry adaptedto receive and downconvert a plurality of OFDM signals, said pluralityof OFDM symbols comprising a first plurality of subcarriers, saidplurality of subcarriers being defined in a soft handoff zone comprisinga first dimension of subchannels, said subchannels comprising said firstplurality of subcarriers; and a second dimension of divided andmultiplexed OFDM symbols; said first plurality of subcarriers in saidsoft handoff zone having identical subchannel definition; FourierTransform (FT) logic adapted to provide a FT on each of said firstplurality of subcarriers to generate a plurality ofdivision-multiplexing coded signals, and decoder logic adapted toprovide division-multiplexing decoding on the plurality ofdivided-multiplexed coded signals to recover data from a base station.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The invention and the illustrated embodiments may be better understood,and the numerous objects, advantages, and features of the presentinvention and illustrated embodiments will become apparent to thoseskilled in the art by reference to the accompanying drawings. In thedrawings, like reference numerals refer to like parts throughout thevarious views of the non-limiting and non-exhaustive embodiments of thepresent invention, and wherein:

FIG. 1 is a block representation of a cellular communication system;

FIG. 2 is a block representation of a base station according to oneembodiment of the present invention;

FIG. 3 is a block representation of a subscriber station according toone embodiment of the present invention;

FIG. 4 is a logical breakdown of an OFDMA transmitter architectureaccording to one embodiment of the present invention;

FIG. 5 is a logical breakdown of an OFDMA receiver architectureaccording to one embodiment of the present invention;

FIG. 6 (a) depicts an example of an OFDM symbol structure in timedomain;

FIG. 6 (b) shows an example of a basic structure of an OFDMA symbol infrequency domain;

FIG. 7 shows an example of subchannel arranged in frequency domain;

FIG. 8 shows a time plan for the OFDMA frame structure in time divisionduplex (TDD) mode;

FIG. 9 (a) shows an example of a cluster;

FIG. 9 (b) shows an example of a tile;

FIG. 10 (a) illustrates an OFDMA frame with multiple zones;

FIG. 10 (b) illustrates an OFDMA frame with a soft handoff zone;

FIG. 11 shows a block representation of a cellular communication systemwith a soft handoff zone;

FIG. 12 is a block representation of a cellular communication systemwith different permutations arrangement and an additional identicalpermutation for soft handoff;

FIG. 13 is an example of interference avoidance in an OFDMA system inaccordance with one embodiment of the present invention;

FIG. 14 shows is an example of a multi-input, multi-output OFDMA systemin accordance with an aspect of the present invention;

FIG. 15 (a) illustrates a general expression for macro-diversity MIMOoperation;

FIGS. 15 (b), (c) and (d) illustrate examples of expressions ofmacro-diversity MIMO operation; and

FIG. 16 shows examples of allocation of subcarriers in macro-diversityMIMO operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to some specific embodiments of theinvention including the best modes contemplated by the inventors forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

In this specification and the appended claims, the singular forms “a,”“an,” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

The term “subscriber station” is intended to include any device whichmay provide connectivity between subscriber equipment and a base station(BS). A subscriber station may be fixed, or mobile. When the subscriberstation is mobile, the speed of its mobile carrier should be apparent toa person skilled in the art, for example, the speed of an automobile, anaircraft or a satellite. The term “base station” is intended to includegeneralized equipment set providing connectivity, management, andcontrol of the subscriber station (SS). The term “protocol data unit”(PDU) is intended to describe a data unit exchanged between peerentities of the same protocol layer. The term “service data unit” (SDU)is intended to describe a data unit exchanged between two adjacentprotocol layers.

With reference to FIG. 1, in a wireless communication system 100 a basestation controller (BSC) 102 controls base stations (BS) 104 withincorresponding cells 106. In general, each base station 104 willfacilitate communications with subscriber stations 108, which are withinthe cell 106 associated with the corresponding base station 104. As asubscriber station 108 moves from a first cell 106 a to a second cell106 b, communications with the subscriber station 108 transition fromone base station 104 to another. The term “handoff” is generally used torefer to techniques for switching from one base station 104 to anotherduring a communication session with a subscriber station 106. The basestations 104 cooperate with the base station controller 102 to ensurethat handoffs are properly orchestrated, and that data intended for thesubscriber station 108 is provided to the appropriate base station 104currently supporting communications with the subscriber station 108.

Handoffs are generally characterized as either hard or soft. Hardhandoffs refer to handoffs where the transition from one base station104 a to another base station 104 b is characterized by the first basestation 104 a stopping communications with the subscriber station 108 bat the precise time when the second base station 104 b beginscommunications with the subscriber station 106. Unfortunately, hardhandoffs are prone to dropping communications, and have proven to besufficiently unreliable. Soft handoffs are characterized by multiplebase stations 104 simultaneously communicating with a subscriber station108 during a handoff period. Identical information may be transmitted tothe subscriber station 108 from different base stations 104, and thesubscriber station 108 attempts to receive signals from both basestations 104 a and 104 b until the base station 104 b to which thesubscriber station 108 b is transitioning is deemed capable of takingover communications with the subscriber station 108. It should beapparent to a person skilled in the art that more than two base stationscan participate in a soft handoff, referring to FIG. 1, for example, allbase stations 104 a, 104 b, and 104 c may participate in a soft handoff.

In FIG. 1, a handoff area 110 is illustrated at the junction of threecells 106, wherein a subscriber station 108 b is at the edge of any oneof the three cells 106 and could potentially be supported by any of thebase stations 104 a, 104 b and 104 c within those cells 106 a, 106 b and106 c. The present invention provides a method and architecture forfacilitating soft handoff in an orthogonal frequency divisionmultiplexing access (OFDMA) wireless communication environment.Orthogonal frequency division multiplexing access (OFDMA) allowsmultiple users, for example subscriber station 108 a and 108 b, totransmit simultaneously on the different subcarriers per OFDM symbol. InOFDMA/TDMA, subcarriers within OFDM symbols are divided by OFDMA methodin frequency domain into subsets of subcarriers, which is termed asubchannel. These subchannels are the basic allocation unit. Eachsubchannel may comprise the subcarriers from a plurality of OFDMsymbols. The subchannel may therefore be spread over the entirebandwidth. Therefore, in the OFDMA/TDMA embodiment, OFDM symbols areshared both in time and in frequency (by subchannel allocation) betweendifferent users. As it will be described later, an SHO zone having thesame subchannel definition, for example, permutation code could bedefined to facilitate the handoff, to provide RF combining, to reduceinterference; and to provide selection combining.

A high level overview of the subscriber stations 108 and base stations104 of the present invention is provided prior to delving into thestructural and functional details of the preferred embodiments. Withreference to FIG. 2, a base station 104 configured according to oneembodiment of the present invention is illustrated. The base station 104generally includes a control system 202, a baseband processor 204,transmit circuitry 206, receive circuitry 208, multiple antennas 210,and a network interface 212. The receive circuitry 208 receives radiofrequency signals bearing information from one or more remotetransmitters provided by subscriber stations 108 (illustrated in FIG.3). Preferably, a low noise amplifier and a filter (not shown) cooperateto amplify and remove broadband interference from the signal forprocessing. Downconversion and digitization circuitry (not shown) willthen downconvert the filtered, received signal to an intermediate orbaseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 204 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 204 is generallyimplemented in one or more digital signal processors (DSPs). Thereceived information is then sent across a wireless network via thenetwork interface 212 or transmitted to another subscriber station 108serviced by the base station 104. The network interface 212 willtypically interact with the base station controller and acircuit-switched network forming a part of a wireless network, which maybe coupled to the public switched telephone network (PSTN) or InternetProtocol (IP) network.

On the transmit side, the baseband processor 204 receives digitizeddata, which may represent voice, data, or control information, from thenetwork interface 212 under the control of control system 202, whichencodes the data for transmission. The encoded data is output to thetransmit circuitry 206, where it is modulated by a carrier signal havinga desired transmit frequency or frequencies. A power amplifier (notshown) will amplify the modulated carrier signal to a level appropriatefor transmission, and deliver the modulated carrier signal to theantennas 210 through a matching network (not shown). Modulation andprocessing details are described in greater detail below.

With reference to FIG. 3, a subscriber station 108 configured accordingto one embodiment of the present invention is illustrated. Similarly tothe base station 104, the subscriber station 108 will include a controlsystem 302, a baseband processor 304, transmit circuitry 306, receivecircuitry 308, multiple antennas 310, and user interface circuitry 312.The receive circuitry 308 receives radio frequency signals bearinginformation from one or more base stations 104. Preferably, a low noiseamplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams.

The baseband processor 304 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed in greater detail below. Thebaseband processor 304 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuit(ASIC).

For transmission, the baseband processor 304 receives digitized data,which may represent voice, data, or control information, from thecontrol system 302, which it encodes for transmission. The encoded datais output to the transmit circuitry 305, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are applicable to the present invention.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each subcarrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation requires the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalis required to recover the transmitted information. In practice, theInverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform(DFT) may be implemented using digital signal processing for modulationand demodulation, respectively.

Accordingly, the characterizing feature of OFDM modulation is thatorthogonal carrier waves are generated for multiple bands within atransmission channel. The modulated signals are digital signals having arelatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

With reference to FIG. 4, a logical OFDM transmission architecture isprovided according to one embodiment. Initially, data 402 to betransmitted to a subscriber station 108 is received at the base station104. The data is scrambled in a manner reducing the peak-to-averagepower ratio associated with the data using data scrambling logic 404. Acyclic redundancy check (CRC) for the scrambled data is determined andappended to the scrambled data using CRC logic 406. Next, channel codingis performed using channel encoder logic 408 to effectively addredundancy to the data to facilitate recovery and error correction atthe subscriber station 108. The channel encoder logic 408 may useforward error correction techniques such as ConcatenatedReed-Solomon-convolutional code (RS-CC), block turbo coding (BTC) orconvolutional turbo codes (CTC). The encoded data is then processed byrate matching logic 410 to compensate for the data expansion associatedwith encoding.

Bit interleaver logic 412 systematically reorders the bits in theencoded data to ensure that adjacent coded bits are mapped ontononadjacent subcarriers, thereby minimize the loss of consecutive databits. This is considered the first step of a two step permutation. Allencoded data bits shall be interleaved by a block interleaver with ablock size corresponding to the number of coded bits per allocatedsubchannels per OFDM symbol. The second step ensures that adjacent codedbits are mapped alternately onto less or more significant bits of theconstellation, thus avoiding long runs of lowly reliable bits.

The resultant data bits are mapped into corresponding symbols dependingon the chosen baseband modulation by mapping logic 414. Binary PhaseShift Key (BPSK), Quadrature Amplitude Modulation (QAM), for example,16-QAM and 64-QAM, or Quadrature Phase Shift Key (QPSK), for example,Gray mapped QPSK modulation may be used. When QAM is used, thesubchannels are mapped onto corresponding complex-valued points in a2^(m)-ary constellation. A corresponding complex-valued 2^(m)-ary QAMsub-symbol, c_(k)=a_(k)+jb_(k), that represent a discrete value of phaseand amplitude, where −N≦k≦N, is assigned to represent each of thesub-segments such that a sequence of frequency-domain sub-symbols isgenerated.

Each of the complex-valued, frequency-domain sub-symbols c_(k) is usedto modulate the phase and amplitude of a corresponding one of 2N+1subcarrier frequencies over a symbol interval T_(s).

The modulated subcarriers are each modulated according to a sine x=(sinx)/x function in the frequency domain, with a spacing of 1/T_(s) betweenthe primary peaks of the subcarriers, so that the primary peak of arespective subcarrier coincides with a null the adjacent subcarriers.Thus, the modulated subcarriers are orthogonal to one another thoughtheir spectra overlap.

The symbols may be systematically reordered to further bolster theimmunity of the transmitted signal to periodic data loss caused byfrequency selective fading using symbol interleaver logic 416. For thispurpose, specific Reed-Solomon permutation may be used to make thesubchannels as independent as possible from each other. The independenceof the subchannel allocation gives maximum robustness and statisticallyspreading interference between neighboring cells as well as neighboringcarriers between two channels and statistically spreading theinterference inside the cell.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. The STC encoder logic418 will process the incoming symbols and provide n outputscorresponding to the number of transmit antennas 210 for the basestation 104. The control system 202 and/or baseband processor 204 willprovide a mapping control signal to control STC encoding. At this point,assume the symbols for the n outputs are representative of the data tobe transmitted and capable of being recovered by the subscriber station108.

For the present example, assume the base station 104 has two antennas210 (n=2) and the STC encoder logic 418 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 418 is sent to a corresponding IFFT processor 420,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing alone or in combination with otherprocessing described herein. The IFFT processors 420 will preferablyoperate on the respective symbols using IDFT or like processing toeffect an Inverse Fourier Transform. The output of the IFFT processors420 provides symbols in the time domain.

It should be apparent to a person skilled in the art that the STCencoder may be a space time transmit diversity (STTD) encoder or aspatial multiplexing (SM) encoder employing, for example, Bell LabsLayered Space-Time (BLAST).

The time domain symbols are grouped into frames, which are associatedwith prefix and pilot headers by like insertion logic 422. Bach of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 424. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 426 and antennas 210. Notably, thetransmitted data is preceded by pilot signals, which are known by theintended subscriber station 108 and implemented by modulating the pilotheader and scattered pilot subcarriers. The subscriber station 108,which is discussed in detail below, will use the scattered pilot signalsfor channel estimation and interference suppression and the header foridentification of the base station 104.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a subscriber station 108. Upon arrival of thetransmitted signals at each of the antennas 310 of the subscriberstation 108, the respective signals are demodulated and amplified bycorresponding RF circuitry 502. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry504 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 506 to control the gain of the amplifiers in the RFcircuitry 502 based on the received signal level.

Preferably, each transmitted frame has a defined structure having twoidentical headers. Framing acquisition is based on the repetition ofthese identical headers. Initially, the digitized signal is provided tosynchronization logic 508, which includes coarse synchronization logic510, which buffers several OFDM symbols and calculates anauto-correlation between the two successive OFDM symbols. A resultanttime index corresponding to the maximum of the correlation resultdetermines a fine synchronization search window, which is used by thefine synchronization logic 512 to determine a precise framing startingposition based on the headers. The output of the fine synchronizationlogic 512 facilitates frame acquisition by the frame alignment logic514. Proper framing alignment is important so that subsequent FFTprocessing provides an accurate conversion from the time to thefrequency domain. The fine synchronization algorithm is based on thecorrelation between the received pilot signals carried by the headersand a local copy of the known pilot data. Once frame alignmentacquisition occurs, the prefix of the OFDM symbol is removed with prefixremoval logic 516 and a resultant samples are sent to frequency offsetand Doppler correction logic 518, which compensates for the systemfrequency offset caused by the unmatched local oscillators in thetransmitter and the receiver and Doppler effects imposed on thetransmitted signals. Preferably, the synchronization logic 508 includesfrequency offset, Doppler, and clock estimation logic 520, which isbased on the headers to help estimate such effects on the transmittedsignal and provide those estimations to the correction logic 518 toproperly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using the FFT processing logic 522.The results are frequency domain symbols, which are sent to processinglogic 524. The processing logic 524 extracts the scattered pilot signalusing scattered pilot extraction logic 526, determines a channelestimate based on the extracted pilot signal using channel estimationlogic 528, and provides channel responses for all subcarriers usingchannel reconstruction logic 530. The frequency domain symbols andchannel reconstruction information for each receive path are provided toan STC decoder 532, which provides STC decoding on both received pathsto recover the transmitted symbols. The channel reconstructioninformation provides the STC decoder 532 sufficient information toprocess the respective frequency domain symbols to remove the effects ofthe transmission channel.

The recovered symbols are placed back in order using the symbolde-interleaver logic 534, which corresponds to the symbol interleaverlogic 416 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 536. The bits are then de-interleaved using bit de-interleaverlogic 538, which corresponds to the bit interleaver logic 412 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 540 and presented to channel decoder logic 542 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 544 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 546 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data.

In OFDM systems, only a single user can transmit on all of thesubcarriers at any given time, and time division or frequency divisionmultiple access may be employed to support multiple users. The majorsetback to this static multiple access scheme is the fact that thedifferent users see the wireless channel differently is not beingutilized. Orthogonal frequency division multiplexing access (OFDMA), onthe other hand, allows multiple users to transmit simultaneously on thedifferent subcarriers per OFDM symbol. In an OFDMA/TDMA embodiment, theOFDM symbols are allocated by TDMA method in the time domain, and thesubcarriers within an OFDM symbols are divided by OFDMA method infrequency domain into subsets of subcarriers, each subset is termed asubchannel. The subcarriers forming one subchannel may, but need not beadjacent. These subchannels are the basic allocation unit. Eachallocation of subchannel may be allocated for several OFDM symbols insuch a way that the estimation of each subchannel is done in frequencyand time. The subchannel may be spread over the entire bandwidth. Thisscheme achieves improved frequency diversity and channel usage withoutthe need for frequency separation between subcarriers. The allocation ofcarriers to subchannel may be accomplished by special Reed-Solomonseries, which enables the optimization and dispersion of interferingsignals inside a cell and between adjacent cells. Therefore, in theOFDMA/TDMA embodiment, OFDM symbols are shared both in time and infrequency (by subchannel allocation) between different users. When theOFDMA is used in the uplink (UL), it allows users to operate withsmaller power amplifiers, at expense of instantaneous data rate. On theother hand it allows allocating dynamically larger amounts of bandwidthto users capable of utilizing it in terms of the link budget. Whenapplied to the downlink (DL), OFDMA allows transmitting to multipleusers in parallel with designated data streams, and may improve the linkbudget of disadvantaged users by allocating to their subchannels alarger fraction of their downlink transmit power.

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as defined in IEEE 806.16-2004 and IEEE 806.16e(available at www.ieee802.org) which are incorporated by reference intheir entireties.

FIG. 6 shows the transmitted OFDM symbols arranged according toincreasing time and increasing subcarrier frequency. The subcarrierfrequencies contained within an OFDM symbol are each represented bycircles. In the time domain, the first two symbols 602 of a frame may bepreamble symbols 610, for example, in the case of a downlink (DL)subframe. Preamble symbols 612 may also be embedded in the frame, forexample, in the case of an uplink (UL) subframe. Data symbols 606 fordata transmission, or scattered pilot symbols for various estimationpurposes 604 are transmitted, depending on the subcarrier frequency,until the next preamble symbols are transmitted. Null subcarriers 608means no transmission, and may be used for guard bands, non-activesubcarriers and the DC subcarrier. The preamble may provide one of thefollowing fundamental operations: fast base station access, base stationidentification and C/I ratio measurement, framing and timingsynchronization, frequency and sampling clock offset estimation andinitial channel estimation. The design of a frame preamble withminimized overhead is critical to maximum spectral efficiency and radiocapacity.

FIG. 6 a shows an example of an OFDMA symbol structure in time domain.OFDMA waveform is created by Inverse-Fourier-Transform. The timeduration 602 is referred to as the useful symbol time T_(b). A copy of asegment 704 (last T_(g)) of the useful symbol period 602, termed cyclicprefix (CP), is copied and appended to the beginning of the usefulsymbol time T_(b) 606, and may be used to collect multipath, whilemaintaining the orthogonality of the tones. Using a cyclic extension,the samples required for performing the FFT at the receiver can be takenanywhere over the length of the extended symbol. This provides multipathimmunity as well as a tolerance for symbol time synchronization errors.

An OFDMA symbol may be characterized by following primitive parameters:the nominal bandwidth (BW); the number of used subcarriers (N_(used)),example, 1703; sampling factor n, which in conjunction with BW andN_(used) determines the subcarrier spacing, and the useful symbol time,and the ratio of CP time T_(g) to useful symbol time T_(b) (G), forexample, ¼, ⅛, 1/16 or 1/32.

Based on the primitive parameters, other parameters could be derived:the FTT size N_(FFT) which is the smallest power of two greater thanN_(used), for the above example of N_(used)=1703, N_(FFT) is 2048;sampling frequency F_(s)=floor (n·8/7·BW/8000)×8000; Subcarrier spacing:Δf=F_(s)/N_(FFT); useful symbol time: T_(b)=1/Δf; CP Time:T_(g)=G·T_(b); OFDMA Symbol Time: T_(s)=T_(b)+T_(g); and sampling time:T_(b)/N_(FFT).

Referring to FIG. 6 b, a basic structure of an OFDMA symbol 610 isdescribed in frequency domain. As discussed in the above, an OFDMAsymbol is made up of subcarriers 612-618, the number of which generallycorrelates to the FFT size used. There may be several subcarrier types;data subcarriers 612, 616, 618 are used for data transmission; pilotsubcarriers 614 are used for various estimation purposes; and nullcarrier has no transmission at all, for guard bands 620 and DC carrier.Guard bands 620 are used to enable the signal to naturally decay andcreate the FFT “brick wall” shaping. In OFDMA, active subcarriers aredivided into subsets of subcarriers, each subset is termed a subchannel.The symbol is divided into subchannels to support scalability, multipleaccess, and advanced antenna array processing capabilities. In FIG. 6(b), three distinct subchannels 612, 616, and 618 are illustrated. Tensand hundreds of subchannels may be implemented. In the downlink, asubchannel may be intended for different (groups of) receivers; in theuplink, a transmitter may be assigned one or more subchannels, severaltransmitters may transmit simultaneously. The subcarriers forming onesubchannel may, but need not be adjacent.

In FIG. 7, each horizontal arrow 702 in the frequency domain 704represents a logical subchannel. The symbol is divided into subchannelsto support scalability, multiple access, and advanced antenna arrayprocessing capabilities. A minimum number of symbols are allocated toone subchannel, this may be accomplished by special Reed-Solomon series,which enable the optimization and dispersion of interfering signalsinside a cell and between adjacent cells. Each subchannel is the basicallocation unit that a user can be allocated. In the time domain 706,OFDM symbols 708 are shown in columns as FIG. 7.

When in a time plan such as the one illustrated in FIG. 8, a slot 802 isdefined as a pair of an OFDM time symbol number and a subchannel logicalnumber. A subchannel is the minimum possible data allocation unit, andits size may vary for uplink and downlink, for full usedsubchannelization (FUSC) and partially used subchannelization (PUSC),and for the distributed subcarrier permutations and the adjacentsubcarrier permutation, between one subchannel by one OFDMA symbol toone subchannel by three OFDMA symbols. For example, in DL and UL PUSCwhich will be discussed below, the DL and UL subframe size and thegranularity of the DL and UL allocations are one by two or one by threeOFDM symbols, respectively. These slots may be referred to as clustersor tiles composing two and three OFDM symbols, respectively.

In OFDMA, a data region is a two-dimensional allocation of a group ofcontiguous subchannels, in a group of contiguous OFDMA symbols. Examplesof data regions are shown in FIG. 8.

The DL-MAP message, if transmitted in the current frame, is the firstMAC PDU in the burst following the FCH. An UL-MAP message followsimmediately either the DL-MAP message (if one is transmitted) or theDLFP. If Uplink Channel Descriptor (UCD) and Downlink Channel Descriptor(DCD) messages are transmitted in the frame, they follow immediately theDL-MAP and UL-MAP messages.

Simultaneous DL allocations can be broadcast, multicast, and unicast andthey can also include an allocation for another base station rather thana serving base station. Simultaneous ULs can be data allocations andranging or bandwidth requests.

There are two major subchannel allocation methods in the downlink:partial usage of subchannels (PUSC) where some of the subchannels areallocated to the transmitter, and full usage of the subchannels (FUSC)where all subchannels are allocated to the transmitter. In FUSC, thereis one set of common pilot subcarriers, but in PUSC, each subchannelcontains its own set of pilot subcarriers.

For FUSC in the downlink 810, the pilot tones are allocated first; thenthe zero carriers, then all the remaining subcarriers are used as datasubcarriers, which are divided into subchannels that are usedexclusively for data. There are two variable pilot-sets and two constantpilot-sets. In FUSC, each segment uses both sets of variable/constantpilot-sets.

Assuming an FTT size of 2048 is used, each subchannel in FUSC maycomprise 48 subcarriers. The subchannel indices may be formulated usinga Reed-Solomon series, and is allocated out of the data subcarriersdomain. The data subcarriers domain includes 48*32=1536 subcarriers,which are the remaining subcarriers after removing from the subcarrier'sdomain (0-2047), the variable set and the constant set of pilots, guardsubcarriers and the DC subcarrier.

The 1536 data subcarriers are partitioned into groups of contiguoussubcarriers. Each subchannel consists of one subcarrier from each ofthese groups. The number of groups is therefore equal to the number ofsubcarriers per subchannel, N_(subcarrier). The number of thesubcarriers in a group is equal to the number of subchannels,N_(subchannels). The partitioning of subcarriers into subchannels can beexpressed in the following permutation formula.subcarrier(k,s)=N _(subchannels) ·n _(k) +{p _(s) [n _(k) mod N_(subchannels)]+IDcell} mod N _(subchannels)

Wherein subcarrier (k,s) is the subcarrier index of subcarrier n insubchannel s, s is the index number of a subchannel, from the set [0 . .. N_(subchannels)−1], n_(k)=(k+13·s) mod N_(subchannels), where k is thesubcarrier-in-subchannel index from the set [0 . . . N_(subchannels)−1],N_(subchannels) the number of subchannels, p_(s)[j] is the seriesobtained by rotating {PermutationBase₀} cyclically to the left s times,ceil[ ] is the function that rounds its argument up to the next integer,IDcell is an integer ranging from 0 to 31, which identifies theparticular base station segment and is specified by MAC layer, andX_(mod(k)) is the remainder of the quotient X/k (which is at most k−1).

For PUSC in the downlink or in the uplink 810, the set of usedsubcarriers is first partitioned into subchannels, and then the pilotsubcarriers are allocated from within each subchannel.

In a downlink using PUSC, a symbol is first divided into basic clustersas illustrated in FIG. 9 (a). Pilots 906 and data carriers 908 areallocated within each cluster 902 904. For an OADM symbol of FFT size2048, the number of used subcarriers, after subtracting the guardsubcarriers (367), is 1681. Each cluster may have 14 subcarriers for atotal of 120 clusters. For the 60 subchannels the allocation ofsubcarriers is as following:

1) Dividing the subcarriers into 120 physical clusters containing 14adjunct subcarriers each (starting from carrier 0).

2) Renumbering the physical clusters into logical clusters using thefollowing formula (As illustrated in FIG. 8, the first PUSC zone of thedownlink, the default IDcell is 0):LogicalCluster=RenumberingSequence((PhysicalCluster+13*IDcell)mod 120)

3) Dividing the clusters into six major groups.

4) Allocating carriers to subchannel in each major group is performed byfirst allocating the pilot carriers within each cluster, and then takingall remaining data carriers within the symbol and using the sameprocedure as described above.

Referring to FIG. 8, an uplink 812 using PUSC, following a downlink 810may also support up to three segments. For an OFDM symbol with FFT size2048, a burst in the uplink may be composed of three time symbols andone subchannel, the three time symbols and one subchannel is termed atile. Within each burst, there are 48 data subcarriers and 24fixed-location pilot subcarrier, a total of 70 subchannels may besupported. The subchannel is constructed from six uplink tiles, eachtile has four subcarriers per symbol. FIG. 9 (b) shows the structure ofa tile with data subcarrier 912 and pilot subcarrier 910.

The permutation PUSC in UL is based on the allocation of tiles tosubchannels through following steps:

1) Divide the 420 tiles into six groups, containing 70 adjacent tileseach.

2. Choose six tiles per subchannel based onTile(s,n)=70·n+(Pt[(s+n)mod 70]+UL_IDcell)mod 70

wherein n is the tile index 0 . . . 5, Pt is the tile permutation, s isthe subchannel number; UL_IDcell is an integer value in the range 0 . .. 69, which is set by the MAC layer.

After allocating the tiles for each subchannel the data subcarriers persubchannel are allocated as follows:

1) After allocating the pilot carriers within each tile, indexing thedata subcarriers within the subchannels is performed starting from thefirst symbol at the lowest subcarrier from the lowest tile andcontinuing in an ascending manner throughout the subcarriers in the samesymbol, then going to next symbol at the lowest data subcarrier, and soon. Data subcarriers shall be indexed from 0 to 47.

2) The allocation of the subcarriers is as follows:subcarrier(n,s)=(n+13·s)mod N _(subcarriers)

wherein n is a running index 0 . . . 47, s is the subchannel number,N_(subcarriers) is the number of subcarriers per subchannel.

There are two main types of subcarrier permutations: distributed andadjacent. In general, distributed subcarrier permutations perform wellin mobile applications while adjacent subcarrier permutations can beproperly used for fixed, portable, or low mobility environments.

OFDMA DL and UL subframes start in DL and UL PUSC mode, respectively. InDL PUSC, subchannels may be divided and assigned to three segments thatcan be allocated to sectors of the same cell. A sector of a cell may beportioned through means known to a Person skilled in the art, forexample, through directional beam.

The available OFDMA subchannels may be divided into subset for deployinga single instance of the MAC, the subset is called a segment. A segmentmay include all available subchannels. In PUSC, for example, any segmenthas at least 12 subchannels. Therefore, a downlink may be divided into athree segments and a preamble structure which begins the transmission.The preamble subcarriers at the beginning of downlink may be alsodivided into three carrier-sets, each of them may be used by one of thesegments in the following manner: segment 0 uses preamble carrier-set 0;segment 1 uses preamble carrier-set 1; and segment 2 uses preamblecarrier-set 2.

Permutation zone is a number of contiguous OFDMA symbols, in the DL orthe UL, that use the same permutation formula. The DL subframe or the ULsubframe may contain more than one permutation zone. An OFDMA frame mayinclude multiple zones as illustrated in FIG. 10. Although the zones inFIG. 10 are shown as vertical columns spanning all the subchannellogical numbers, it should be apparent to a person skilled in the artthat a permutation zone may also have other irregular shapes on a TDDtime plan such as the one illustrated in FIG. 8.

FIG. 10 (a) illustrates zone switching within the DL and UL subframes.The switching is performed using an information element included inDL-MAP and UL-MAP. DL and UL subframes both start in PUSC mode wheregroups of subchannels are assigned to different segments by the use ofdedicated FCH messages. The PUSC subcarrier allocation zone 1004 can beswitched to a different type of subcarrier allocation zone through adirective from the PUSC DL-MAP 1004. FIG. 10 (a) shows the zoneswitching from the perspective of a PUSC segment. In FIG. 10 (a), thefirst zone PUSC contains FCH and DL-MAP 1004 is followed with anotherpossibly data PUSC zone with a parameter “DL_PermBase X” 1006. A FUSCzone for another sector/cell with “DL_PermBase Y” 1008 is allocatednext, followed by an FUSC zone for “DL_PermBase Z” 1010. A switching toFUSC “DL_PermBase M” 1012 can then be planned. Optional PUSC, FUSC 922,and AMC 924 zones in DL subframes and optional PUSC 1020 and AMC zonesin UL subframes can be similarly scheduled. Allocation of AMC zonessupport simultaneously fixed, portable, and nomadic mobility users alongwith high mobility users.

Referring to FIG. 1, when a subscriber station 108 b enters the networkinitially, coarse synchronization correlation is performed based on thepreamble header in the time domain to determine a coarse synchronizationlocation. At the coarse synchronization location, a fine synchronizationsearch window is identified. An FFT is computed, and the system switchesto a common synchronization channel to perform fine synchronizationwithin the fine synchronization search window. The strongest correlationpeaks are then identified, and the relevant time index are used as thecandidate timing synchronization positions. An FFT is computed at eachcandidate timing synchronization position, and the system switches tothe pilot channel.

The pseudo-noise (PN) sequences for all base stations 104 arecorrelated, and correlation peaks are selected to define an indexcorresponding to all candidate timing synchronization positions. The ORsfor these base stations 104 are identified. The base station withhighest CIR is selected as the serving base station, and the basestations 104 with CIRs greater than a given threshold are also selectedfor an active set list. If more than one base station 104 is on theactive set list, the soft handoff procedures of the present inventionare initiated. The FFT is then computed and the fine synchronization isprovided using the PN code for each of the selected base station(s) 104.

In operation, downlink communications from a base station 104 to asubscriber station 108 are initiated by the subscriber station 108. Eachsubscriber station 108 constantly measures all of the possible pilotsignal strengths of transmissions from adjacent base stations 104,identifies the strongest pilot signals, and compares them against adefined threshold. If the pilot signal strength for a base station 104exceeds the defined threshold, that base station 104 is added to anactive set list. Each subscriber station 108 will notify the basestations 104 of their active set lists. If there is only one basestation 104 in the active set list, that base station 104 is singled outto service the subscriber station 108. If there is more than one basestation 104 on the active set list, a soft handoff is enabled betweenthose base stations 104. The soft handoff condition will continue untilonly one base station 104 is on the active set list, wherein the lonebase station 104 will continue to serve the subscriber station 108.During soft handoff, all base stations 104 on the active set list willfacilitate communications with the subscriber station 108 as definedbelow. Preferably, the base station controller 102 keeps track of all ofthe active set lists for the respective subscriber stations 108. Thesubscriber station 108 will keep track of their individual active setlists.

Accordingly, by providing the set list to the base station controller102 and the servicing base station 104, the subscriber station 108identifies the sole servicing base station 104 or triggers a softhandoff (SHO) mode when multiple base stations appear on the active setlist. During a SHO mode, the base station controller 102 multicasts datapackets intended for the subscriber station 108 to each of the basestations 104 on the active set list through macro-diversity.Multicasting indicates that each data packet is sent to each basestation 104 on the active set list for transmission to the subscriberstation 108. Alternatively, “non-redundant transmission” i.e. the datapackets are divided into sub-packets in some manner and each sub-packetis sent to one of the base stations 104 on the active set list fortransmission to the subscriber station 108, may also be implemented.

Exemplary process for identifying base station 104 to place them on theactive list, as well as exemplary flow of an active SHO process has beendescribed in PCT Application PCT/IB03/00153, filed in January 2003, andpublished as WO03/081938 on Oct. 2, 2003, which is incorporated hereinby reference in its entirety.

In accordance with one embodiment of the present invention, there is anSHO zone defined for the soft handoff mode. Referring to FIG. 10 (b), anSHO zone 1022 is illustrated as a zone in the DL subframe, and an SHO1024 as a zone in the UL subframe in time plan diagram. For PUSC, FUSC,AMC, optional FUSC or any other zones apparent to a person skilled inthe art, such as Tile Usage of Subchannels (TUSC), in DL can be appliedto SHO. Similarly, the PUSC, Optional PUSC or AMC zones in UL can beapplied to SHO. Hence, the subscriber stations on the active list usethe same subchannel definition, for example, permutation in OFDMA asdefined in the SHO zone. This soft hand-off based macro-diversitytransmission enables the concurrent transmissions to a target subscriberstation from multiple base stations. Advantageously, this allows thesubscriber station to exploit the macro-diversity gain to enhance bothhigh speed user bit rate coverage and seamless hand-off of real-timeservice.

In one embodiment of the present invention, the DL SHO basedmacro-diversity provides RF combining. The DL macro-diversity RFcombining turns interference into signal, hence significantly improvesCIR and increases data throughput. The DL macro-diversity RF combiningis transparent to the subscriber station reception operation, andenables simple subscriber station backward compatibility.

An SHO zone may be defined by the OFDMA downlink STC_ZONE_IE by settingthe IDcell=0. For the SHO-base stations joint transmission, for the STCcapable subscriber station, the total N antennas of SHO-base stationsconstitute an antenna pool. A pre-determined antenna selection formulamay be used.

The MIMO pilot transmission for two-antenna transmission in PUSC andFUSC modes may follow the arrangement, for example, described in FIGS.245 and 8.4.8.1.2.1.2 of IEEE802.16-2004, respectively.

The MIMO pilot transmission for four-antenna transmission in PUSC andFUSC modes may follow the arrangement, for example, described in FIGS.251 and 8.4.8.2.2 of IEEE802.16-2004, respectively. The un-selectedantennas may be set to the null transmission.

Subscriber station demodulates signal in the same procedure as innon-SHO mode if it does not receive MIMO_in_another_BS_IE( ) orMacro_MIMO DL Basic IE( ). The same data are transmitted from multiplebase stations in the same data regions.

As described below, subscriber station may perform RF or diversitycombining. Subscriber station may further perform soft data combiningwhen it receives MIMO_in_another_BS_IE( ). In this case, the same dataare transmitted in the same or different data regions.

Subscriber station demodulates signal in the same procedure as innon-SHO mode, it performs soft combining for those data regions with thesame packet index when it receives Macro_MIMO DL Basic IE( ). Thisscheme benefits from combination of RF, diversity combining and softdata combining.

Table 1 is shows an example of IMO_in_another_BS_IE( ).

Size Syntax (Bits) Notes IMO_in_another_BS_IE ( ) {  Extended-2 DIUC 4MIMO in another BS IE = OxO4  Length 8 variable  Segment 2 Segmentnumber  Used subchannels 6 Used subchannels at other BS Bit #0: 0-11 Bit#1: 12-19 Bit #2: 20-31 Bit #3: 32-39 Bit #4: 40-51 Bit #5: 52-59 IDCell5 Cell ID of other BS Num-Region 4 for (i=0; i<Num_Region; i++) { Matrix_indicator 2 STC matrix (see 8.4.8.1.4 of IEEE802.26-2004) STC =STC mode indicated in the latest STC_Zone_IE ( ). if (STC = 0b01) andAnt23 ==0){  0b00 = Matrix A  0b01 = Matrix B  0b10 = Matrix C  0b11 =Reserved } elseif (STC == 0b01 and Ant23 ==1) or (STC == 0b10){  0b00 =Matrix A  0b01 = Matrix B  0b10 = Matrix C  0b11 = Reserved } else {0b00-0b11 = Reserved }  OFDMA Symbol offset 8  Subchannel offset 6 Boosting 3 Refer to Table 273.  No. OFDMA Symbols 7  No. subchannels 6 Num_layer 2  For  (i=0;  j<Num_Layer; j++){   If (INC_CID == I){ — —   CID 16  —   } — —   Layer_index 2 —   DIUC 4 0-11 burst profiles   }  }  }

Referring to FIG. 11, to achieve RF combining in PUSC with samepermutation and the same connection identifier (CID) in each cell 106,an SHO Zone 1022 with a common IDcell M 1102 may be used. the basestations in an active set 1104 transmit the same data in the data regionas defined, for example in a downlink map (DL_MAP). A CID may be definedas a 16 bit value that identifies a connection to equivalent peers inthe MAC of the base station and subscriber station. Subscriber station108 b uses the same procedure to decode data as in non-SHO mode. Signalsfrom multiple base stations are energy combined at subscriber station108 b if the relative delays within active base stations are smallerthan a communication signal prefix or a pre-defined value. Base stationswith relative delays larger than the prefix or the pre-defined value canturn off their transmission in this data region to achieve interferenceavoidance. Base stations preferably coordinate their schedulers suchthat different subscriber stations use different data regions.

A second embodiment to achieve RF combining in PUSC is illustrated inFIG. 12. Each BS PUSC allocation is based on a default arrangement 1204,1206, 1208, therefore each sector PUSC is allocated based on differentpermutations. However, collaborative transmission base stations in theactive set each transmit additional PUSC segments 1210 with thesubchannel definition, for example, permutation identical to the anchorsegments.

To achieve RF in a DL FUSC with same subchannel definition, for example,permutation and the same CID in each cell, as described above, in a SHOZone where a common IDcell is used, the base stations in an active settransmit the same data in a data region defined, for example, in DL_MAP.Subscriber station uses the same procedure to decode data as in non-SHOmode. Signals from multiple base stations are energy combined if therelative delays within active base stations are smaller than the prefixor a pre-defined value. Base stations with relative delays larger thanthe prefix or the pre-defined value can turn off their transmission inthis data region to achieve interference avoidance. Base stationspreferably coordinate their schedulers such that different subscriberstations use different data regions and to avoid interference to channelestimation.

In another embodiment of the present invention, the DL SHO basedmacro-diversity provides soft combining. To achieve the soft combining,same DL transmission format is mapped onto different PHY subchannels andtransmitted over different macro-diversity branches. The subscriberstation separately demodulates different macro-diversity versions andcombines received packets at Log Likelihood Ratio (LLR) level, which isthe natural logarithm of the probabilities that the informationtransmitted assumes its two possible values. Therefore, the softcombining of macro-diversity gain is achieved. As an example, in PUSCwith same permutation and same CID in each cell, additional segment maybe used. Base stations in an active set transmit the same data and usethe same data randomizer. Subscriber station demodulates signals fromeach base station, combines soft bits from each base station, and thendecodes the data based on the combined soft bits, e.g. LLR combining. Inthis case, subscriber station processes the data from all serving basestations separately and may apply maximal ratio combining, for example.Each base station can use a different subchannel. Since all serving basestations should have the same CID, a centralized controller or MAPmodification may be used to resolve the CID collision between servingbase stations.

In another embodiment of the present invention, the DL SHO basedmacro-diversity provides interference avoidance for PUSC/FUSC with samepermutation and the same CID in each cell.

In an SHO Zone where a common IDcell is used, base stations in an activeset transmit the same data in the data region defined in DL_MAP.Referring to FIG. 13, base station 104 c can simply turn off itstransmission in this data region to achieve interference avoidance.Subscriber station uses the same procedure to decode data as in non-SHOmode, with signals from multiple base stations being energy combined orsoft combined if the relative delays within active base stations aresmaller than the prefix or a predefined value. Base stations withrelative delays larger than prefix or a predefined value can turn offtheir transmission in this data region to achieve interferenceavoidance. Base stations preferably coordinate their schedulers suchthat different subscriber stations use different zones 1302, 1304.

In another embodiment of the present invention, the DL SHO basedmacro-diversity provides selection combining for PUSC/FUSC withdifferent permutations or different CIDs in each cell.

Referring to FIG. 11, base stations in an active set transmit the samedata. Subscriber station demodulates data from each base station,selects one successfully decoded data from, for example, base station104 c. In UL macro-diversity SHO, similar to the downlink case, for PUSCwith same subchannel definition, for example, permutation in each cell,the soft handover zone is used. The subscriber station may transmit datato serving base station but other base stations in active set can alsoreceive data, which makes the selection diversity possible. Oneadvantage of this scheme is that it does not require additionalcomplexity to subscriber station. Subscriber station transmits the datain the same way as in the non-SHO case. Also, selection diversity gaincan be achieved.

For PUSC with different subchannel definition, for example, permutationin each cell, all base stations in active set assign uplink data regionto subscriber station, and subscriber station sends the uplink data toall active base stations according to the allocation information of eachbase station. Subscriber station preferably supports all the UL-MAP fromthe base stations in the active set. Another possible implementationinvolves a serving or anchor base station sending the UL-MAP with theallocation information of itself and other base stations in the activeset. In this case, subscriber station only needs to see the UL-MAP fromthe anchor base station.

Table 2 provides a summary of SHO based macro-diversity transmissionschemes.

TABLE 2 RF Soft Interference Selection Combining Combining Joint MIMOAvoidance combining Configurations PUSC/FUSC PUSC/FUSC PUSC/FUSCPUSC/FUSC PUSC/FUSC Common Yes No Yes Yes No Permutation Common CID YesYes Yes Yes No DL Yes Yes Yes Yes Yes UL No No No No Yes Subscriberstation Yes No No Yes No Backward Compatible Coverage benefit LLRInterference LLR Interference Selective combining level reductioncombining with level reduction diversity with with same onlyinterference only same level level level reduction interferenceinterference

In MIMO system, base stations in an active set may preferably transmitthe same data in the data region defined in DL_MAP in a SHO Zone where acommon IDcell is used. The total N antennas of SHO-base stationsconstitute an antenna pool. The anchor base station selects certainnumbers of antennas from the antenna pool, and decides MIMO transmissionmode based on subscriber station capability and channel condition, forexample. The antenna selection can be varied from subchannel tosubchannel to maximize spatial diversity. A pre-determined antennaselection formula can be used. For FUSC, anchor base station maycoordinate schedulers to avoid channel estimation interference. For aparticular subchannel, the allocated antennas in the base stations inthe active set may concurrently transmit the data for the same packetwith the same CID and use the same data randomizer. The subscriberstation receives the RF-combined MIMO signal from the same data regionand demodulates it, and then decodes the packet based on the combinedsoft bits between the different data region.

The source data in the different antennas may also be different, in thiscase, the macro-diversity MIMO scheme intends to achieve higher cellthroughput, or to decode the packet based on the combined soft bitsbetween the different data regions.

In a MIMO system, after receiving transmitted data encoded in space-time(STC) code the subscriber stations provide corresponding STC decoding torecover the transmitted data. The STC coding may be eitherspace-time-transmit diversity (STTD), space-frequency-transmit diversity(SFTD) or space multiplexing (SM) coding. STTD coding encodes data intomultiple formats and simultaneously transmits the multiple formats withspatial diversity (i.e. from antennas at different locations). SM codingseparates data into different groups and separately encodes andsimultaneously transmits each group. Other coding schemes will berecognized by those skilled in the art. The subscriber station willseparately de-modulate and decode the received data from each basestation, and then combine the decoded data from each base station torecover the original data.

There are three levels of macro-diversity MIMO operations which may becombined to improve both the overall handoff performance and the cellthroughput.

In the case of macro-diversity MIMO with RF combining, the packet beingdelivered to SHO subscriber station is duplicated and all or someantennas in the antenna pool formed with SHO base stations transmit thedata for the same packet in the same data region such as a subchannel.

In the case of macro-diversity MIMO with diversity combining, the datafor the same packet is transmitted through another set of antennas inanother data region with the same size, and these two can besoft-combined in order to achieve diversity combining.

In the case of macro-diversity MIMO with data rate enhancement, the datafor the different packet is transmitted through another set of antennasin the same or another data region, and these two can be separatelydecoded in order to achieve data rate increase. Note that for thisscheme, two data regions shall be different.

For a certain subscriber station, these three schemes may be implementedsimultaneously. This macro-diversity MIMO enhancement operation may alsobe transparent to subscriber station, as each SHO subscriber station maynot know which base stations are transmitting in order to decode thetransmitted data.

A general expression for macro-diversity MIMO operation is show in FIG.15 (a), wherein N is the total number of antennas in the antenna poolused for macro-diversity MIMO and K is the number of allocated frequencyregion for the subscriber station. The ‘0’ in the matrix indicates ‘nodata transmission’ and ‘S’ is ‘data transmission’. S=[a or a′ or b or b′. . . k or k′], N=(number of base stations)×(number of antennas per basestation).

Macro-diversity MIMO with RF combining is described in FIG. 15 (b). Inthis example, after RF combining from three base stations, the receiveddata is further STC decoded. Antenna 1 and antenna 2 form a pair ofantennas from the antenna pool.

FIG. 15 (c) shows an example for macro-diversity MIMO with diversitycombining and STC decoding.

FIG. 15 (d) is an example for macro-diversity MIMO with data rateenhancement combined with STC.

In DL SHO macro-diversity MIMO applications, several base stations maybe selected by a subscriber station to perform collaborativetransmissions. The collaborative transmission can be considered asinterference avoidance, multicast and MIMO transmission with spacemultiplexing or space time coded transmission format.

As an example shown in Table 3, among 5 base stations, in the activeset, each base station has various number of transmit antennas, forexample, BS-1 has only two antennas while BS-2 has four antennas. Forexample, A(2,1) denotes Antenna-1 of base station 2. This base stationconfiguration is used in the following examples.

TABLE 3 BS-1 BS-2 BS-3 BS-4 BS-5 Antenna-1 A(1, 1) A(2, 1) A(3, 1)A(4, 1) A(5, 1) Antenna-2 A(1, 2) A(2, 2) — A(4, 2) A(5, 2) Antenna-3 —A(2, 3) — A(4, 3) — Antenna-4 — A(2, 4) — — —

The antennas can be treated as antenna pool resource. Both open loop andclosed loop solutions may be used for different space time codingformats.

In open loop transmission where no feedback channel is present,preferably involves coordination at network level to arrange the antennatransmission format. There may exist a deterministic antenna selectionrule for open loop transmission. Referring to FIG. 14, a two-branchtransmission between the subscriber station 108 b and base stations 104b, 104 c is illustrated.

The open loop macro-diversity transmission which may be space timetransmit diversity (STTD) or spatial multiplexing (SM) is shown in Table4, where synchronous packet streams are delivered to active set basestation, in this case BS-1 and BS-2. As can be seen from Table 4, MIMOantenna #1 and MIMO antenna #2, are defined by alternating transmittingantennas from different base stations.

TABLE 4 OFDM Symbol/ MIMO MIMO Subcarrier BS-1 Antenna # BS-2 Antenna #1 A(1, 1) 1 A(2, 1) 2 2 A(1, 2) 1 A(2, 2) 2 3 A(1, 1) 1 A(2, 3) 2 4 A(1,2) 1 A(2, 4) 2

The transmit matrix definition for the MIMO antennas in Table 4 isdefined in Table 5. FIG. 16 a shows an exemplary mapping of subcarriers,the position of the subcarriers 1601 in relation to the antennas 1602,1604 are illustrated in relation to the subcarrier index 1606 and OFDMindex 1608.

TABLE 5 OFDM SM (2Rx) STTD (1Rx) Symbol/Subcarrier 1 2 1 2 MIMO Antenna#1 S₁ S₃ S₁ S₂ MIMO Antenna #2 S₂ S₄ −S₂* S₁*

The open loop macro-diversity transmission which may be space timetransmit diversity (STTD) or spatial multiplexing (SM) is shown in Table6, where asynchronous packet streams are delivered to active set basestation, in this case BS-1 and BS-2. As can be seen from Table 5, MIMOantenna #1 to MIMO antenna #2, are defined by alternating transmittingantennas from different base stations.

TABLE 6 OFDM MIMO MIMO Symbol/Subcarrier BS-1 Antenna # BS-2 Antenna # 1A(1, 1) 1 A(2, 1) 3 2 A(1, 2) 2 A(2, 2) 4 3 A(1, 1) 1 A(2, 3) 3 4 A(1,2) 2 A(2, 4) 4

The matrix definition for the MIMO antennas in Table 6 is defined inTable 7, where a single receiving antenna is used for STTD decoding, andin Table 8, where four receiving antennas are used for SM, and twoantennas for STTD. FIG. 16 b shows an exemplary mapping of subcarriers,the position of the subcarriers 1601 in relation to the antennas 1610 to1616 are illustrated in relation to the subcarrier index and OFDM index.

TABLE 7 OFDM STTD (1Rx) Symbol/Subcarrier 1 2 3 4 MIMO Antenna #1 S₁ S₂0 0 MIMO Antenna #2 −S₂* S₁* 0 0 MIMO Antenna #3 0 0 S₃ S₄ MIMO Antenna#4 0 0 −S₄* S₃*

TABEL 8 OFDM SM (4 Rx) STTD (2Rx) Symbol/Subcarrier 1 2 1 2 MIMO Antenna#1 S₁ S₅ S₁ S₂ MIMO Antenna #2 S₂ S₆ −S₂* S₁* MIMO Antenna #3 S₃ S₇ S₃S₄ MIMO Antenna #4 S₄ S₈ −S₄* S₃*

The open loop macro-diversity transmission with three antennas is shownin Table 9, where synchronous packet streams are delivered to active setbase station, in this case BS-1, BS-2 and BS-3. As can be seen fromTable 8, MIMO antenna #1, #2 and #3 are defined by the antennas fromdifferent base stations.

TABLE 9 OFDM MIMO Symbol/ Antenna MIMO MIMO Subcarrier BS-1 # BS-2Antenna # BS-3 Antenna # 1 A(1,1) 1 A(2,1) 2 A(3,1) 3 2 A(1,2) 1 A(2,2)2 A(3,1) 3 3 A(1,1) 1 A(2,3) 2 A(3,1) 3 4 A(1,2) 1 A(2,4) 2 A(3,1) 3

The matrix definition for the MIMO antennas in Table 9 is defined inTable 10, where three receiving antennas are used for SM, and twoantennas for STTD. FIG. 16 c shows an exemplary mapping of subcarriers,the position of the subcarriers 1601 in relation to the antennas 1618,1622, and 1624 are illustrated in relation to the subcarrier index andOFDM index, in this example subcarrier 1620 is not used.

TABLE 10 OFDM Symbol/ SM (3Rx) STTD (2Rx) Subcarrier 1 2 3 1 2 3 4 MIMOAntenna S₁ S₅ S₉ S₁ S₂ 0 0 #1 MIMO Antenna S₂ S₆ S₁₀ −S₂* S₁* S₃ S₄ #2MIMO Antenna S₃ S₇ S₁₁ 0 0 −S₃* S₄* #3

The open loop macro-diversity transmission with three antennas is shownin Table 11, where asynchronous packet streams are delivered to activeset base station, in this case BS-1, BS-2 and BS-3. As can be seen fromTable 8, MIMO antenna #1 to #6 are defined by the antennas fromdifferent base stations.

TABLE 11 OFDM MIMO Symbol/ Antenna MIMO MIMO Subcarrier BS-1 # BS-2Antenna # BS-3 Antenna # 1 A(1,1) 1 A(2,1) 3 A(3,1) 5 2 A(1,2) 2 A(2,2)4 A(3,1) 6 3 A(1,1) 1 A(2,3) 3 A(3,1) 5 4 A(1,2) 2 A(2,4) 4 A(3,1) 6

The matrix definition for the MIMO antennas in Table 11 is defined inTable 12, where three receiving antennas are used for SM, and singleantenna for STTD. FIG. 16 d shows an exemplary mapping of subcarriers,the position of the subcarriers 1601 in relation to the antennas 1626 to1636 are illustrated in relation to the subcarrier index and OFDM index.

TABLE 12 SM OFDM Symbol/ (3Rx) STTD (1Rx) Subcarrier 1 2 1 2 3 4 5 6MIMO Antenna S₁ S₇ S₁ S₂ 0 0 0 0 #1 MIMO Antenna S₂ S₈ −S₂* S₁* 0 0 0 0#2 MIMO Antenna S₃ S₉ 0 0 S₃ S₄ 0 0 #3 MIMO Antenna S₄ S₁₀ 0 0 −S₄* S₃*0 0 #4 MIMO Antenna S₅ S₁₁ 0 0 0 0 S₅ S₆ #5 MIMO Antenna S₆ S₁₂ 0 0 0 0−S₆* S₅* #6

Table 13 is an example of UL control channel for closed loopmacro-diversity transmission with dynamic antenna selection. The closedloop transmission applies a sub-MIMO selection technique to chose bestantenna configuration by intra-base station antenna switching insubstantially the same manner as described above. However, a controlchannel for antenna selection is preferably provided.

TABLE 13 Vector Indexes per Tile Codeword # Tile (0) Tile (1) Tile (2)Tile (3) Tile (4) Tile (5) 0 0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3 33 3 3 3 3 4 4 4 4 4 4 4 5 5 5 5 5 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7

In this example, each subscriber station may have 8 code words for theantenna selection configuration of 3-bit.

Several illustrative combinations of 2-, 3- and 4-branch SHO joint MIMOconfigurations are summarized in Table 14.

TABLE 14 2-BS 3-BS 4-BS MSS-Rx-1 [BS1-1; [BS1-2; [BS1-1; [BS1-2; [BS1-1;[BS1-2; BS2-1] BS2-2] BS2-1] BS2-2] BS2-1] BS2-2] 2x1-STTD 4x1-STTD3x1-STTD 4x1-STTD 4x1-STTD 4x1-STTD (Synch) (Synch) (Synch) (Synch)(Synch) (Synch) MSS-Rx-2 [BS1-1; [BS1-2; [BS1-1; [BS1-2; [BS1-1; [BS1-2;BS2-1] BS2-2] BS2-1] BS2-2] BS2-1] BS2-2] 2x2-SM 4x2-STTD 3x2-STTD4x2-STTD 4x2-STTD 4x2-STTD (Asynch) (Synch) (Synch) (Synch) (Synch)(Synch) MSS-Rx-4 [BS1-1; [BS1-2; [BS1-1; [BS1-2; [BS1-1; [BS1-2; BS2-1]BS2-2] BS2-1] BS2-2] BS2-1] BS2-2] 2x4-SM 4x4-SM 3x4-SM 4x4-SM 4x4-SM4x4-SM (Asynch) (Asynch) (Asynch) (Asynch) (Asynch) (Asynch)

What is claimed is:
 1. A method for facilitating soft handoffs in anorthogonal frequency division multiplexing access (OFDMA) system, themethod comprising: generating a first stream of OFDM symbols at a firstbase station, wherein each of the OFDM symbols includes a common set ofsubcarriers, wherein the first stream of OFDM symbols includes aplurality of zones, wherein each of the zones extends in a timedimension over more than one of the OFDM symbols and includes one ormore subsets of the subcarriers, wherein the zones include a softhandoff zone, wherein a definition of the one or more subsets ofsubcarriers belonging to the soft handoff zone is shared by the firstbase station and one or more additional base stations that areparticipants in a soft handoff with respect to a first subscriberstation; wirelessly transmitting the first stream of OFDM symbols intospace.
 2. The method of claim 1, wherein the definition of the one ormore subsets of subcarriers belonging to each zone is specified by arespective subcarrier permutation, wherein a first subcarrierpermutation corresponding to the soft handoff zone is used by the firstbase station and the one or more additional base stations to define theone or more subsets belonging to the soft handoff zone.
 3. The method ofclaim 2, wherein the first subcarrier permutation is a distributedsubcarrier permutation or an adjacent subcarrier permutation.
 4. Themethod of claim 1, wherein the first subscriber station is located in asoft handoff area, wherein the one or more subsets of subcarriersbelonging to the soft handoff zone are transmitted in a downlinksubframe of an OFDMA frame.
 5. The method of claim 1, wherein the softhandoff zone includes a first data region, wherein the first basestation and the one or more additional base stations simultaneouslytransmit the same data in the first data region to facilitate RFcombining at the first subscriber station.
 6. The method of claim 1,wherein the soft handoff zone includes a plurality of data regions,wherein the first base station and the one or more additional basestations transmit the same data in respectively different ones of thedata regions to facilitate soft combining at the first subscriberstation.
 7. The method of claim 1, wherein the first base station andthe one or more additional base stations are defined by an active listgenerated by the first subscriber station.
 8. The method of claim 1,wherein at least one of said one or more additional base stations turnsoff its transmission in the soft handoff zone to provide interferenceavoidance.
 9. The method of claim 1, wherein said first base station andsaid one or more additional base stations provide a plurality ofantennas to form an antenna pool, wherein the one or more additionalbase stations transmit respective streams of OFDM symbols while thefirst base station is performing said transmitting of the first streamof OFDM symbols, wherein said transmitting the first stream by the firstbase station and said transmitting the respective streams by the one ormore additional base stations form a multi-input multi-output (MIMO)transmission to the first subscriber station.
 10. The method of claim 9,wherein said MIMO transmission is based on space-time coding (STC) orspace-frequency coding (SFC) or space multiplexing coding (SM).
 11. Abase station in an orthogonal frequency division multiplexing access(OFDMA) system, the base station comprising: a processor; and memorystoring program instructions, wherein the program instructions, whenexecuted by the processor, cause the processor to: generate a firststream of OFDM symbols, wherein each of the OFDM symbols includes acommon set of subcarriers, wherein the first stream of OFDM symbolsincludes a plurality of zones, wherein each of the zones extends in atime dimension over more than one of the OFDM symbols and includes oneor more subsets of the subcarriers, wherein the zones include a softhandoff zone, wherein a definition of the one or more subsets ofsubcarriers belonging to the soft handoff zone is shared by the basestation and one or more additional base stations that are participantsin a soft handoff with respect to a first subscriber station; wirelesslytransmit the first stream of OFDM symbols into space.
 12. The basestation of claim 11, wherein the definition of the one or more subsetsof subcarriers belonging to each zone is specified by a respectivesubcarrier permutation, wherein a first subcarrier permutationcorresponding to the soft handoff zone is used by the base station andthe one or more additional base stations to define the one or moresubsets belonging to the soft handoff zone.
 13. The base station ofclaim 12, wherein the first subcarrier permutation is a distributedsubcarrier permutation or an adjacent subcarrier permutation.
 14. Thebase station of claim 11, wherein the first subscriber station islocated in a soft handoff area, wherein the one or more subsets ofsubcarriers belonging to the soft handoff zone are transmitted in adownlink subframe of an OFDMA frame.
 15. The base station of claim 11,wherein the soft handoff zone includes a first data region, wherein thebase station and the one or more additional base stations are configuredto simultaneously transmit the same data in the first data region tofacilitate RF combining at the first subscriber station.
 16. The basestation of claim 11, wherein the soft handoff zone includes a pluralityof data regions, wherein the base station and the one or more additionalbase stations are configured to transmit the same data in respectivelydifferent ones of the data regions to facilitate soft combining at thefirst subscriber station.
 17. The base station of claim 11, wherein thebase station and the one or more additional base stations are defined byan active list generated by the first subscriber station.
 18. The basestation of claim 11, wherein at least one of said one or more additionalbase stations is configured to turn off its transmission in the softhandoff zone to provide interference avoidance.
 19. The base station ofclaim 11, wherein said base station and said one or more additional basestations provide a plurality of antennas to form an antenna pool,wherein the one or more additional base stations are configured totransmit respective streams of OFDM symbols while the base station istransmitting the first stream of OFDM symbols, wherein said transmittingthe first stream by the base station and said transmitting therespective streams by the one or more additional base stations form amulti-input multi-output (MIMO) transmission to the first subscriberstation.
 20. The base station of claim 19, wherein said MIMOtransmission is based on space-time coding (STC) or space-frequencycoding (SFC) or space multiplexing coding (SM).